Aromatic polycarbonate resins are used in a wide variety of industrial fields as a material for various molded articles. Especially for application fields in which high transparency is required, such as lighting covers and protective covers for transmission type displays, the aromatic polycarbonate resins are widely used, making use of excellent transparency typified by a high light transmittance and an extremely low haze. In these application fields, attention is now also paid to flame retardancy at the time of a fire, and a resin composition having high flame retardancy in addition to the above characteristic property is desired.
In order to provide flame retardancy to the aromatic polycarbonate resins, it has been proposed to add a bromine-based compound or a phosphorus-based compound, which is employed for OA equipment and home electric appliances all of which are strongly desired to be flame retarded. A flame retardant as a substitute for a bromine-based or phosphorus-based flame retardant has been developed and used in the above products. The purpose of changing the flame retardant is to suppress the generation of a corrosive gas at the time of molding or to improve the recyclability of a product.
An example of the flame retardant except for the bromine-based and phosphorus-based flame retardants is a silicone compound. A resin composition comprising the silicone compound and an aromatic polycarbonate resin is now under study energetically, and various proposals are made.
For example, there is proposed a method of mixing an alkali (earth) metal salt of a perfluoroalkylsulfonic acid and an organic siloxane having an alkoxy group, vinyl group and phenyl group with a polycarbonate resin (refer to Patent Document 1). There is also provided a method of mixing an alkali metal salt or alkali earth metal salt of a perfluoroalkylsulfonic acid and an organopolysiloxane containing an organosilyl group bonded to a silicon atom via a divalent hydrocarbon group with a polycarbonate resin (refer to Patent Document 2).
There is further proposed a method of mixing a specific petroleum-based heavy oil or pitch and a silicone compound with a resin component (refer to Patent Document 3). There is still further proposed a method of mixing a silicone resin having a unit represented by R2SiO1.0 and a unit represented by RSiO1.5 (R is a hydrocarbon group) and a weight average molecular weight of 10,000 to 270,000 with a non-silicone resin having an aromatic ring (refer to Patent Document 4).
However, most of the above proposed resin compositions are unsatisfactory in terms of transparency and flame retardancy because they cannot achieve UL94 V-0 rating as they drip when they are thin, their molded articles become clouded due to the unsatisfactory dispersion of silicone, or transparency degrades after a moist heat treatment due to the agglomeration of silicone by the moist heat treatment.
To suppress dripping, use of polytetrafluoroethylene having fibril formability is effective. However, when polytetrafluoroethylene is mixed with an aromatic polycarbonate resin, the transparency of a molded article degrades due to incompatibility between polytetrafluoroethylene and the aromatic polycarbonate resin.
There is also proposed a resin composition which comprises an aromatic polycarbonate resin, an organic alkali metal salt and poly(methyl hydrogen siloxane) (refer to Patent Document 5). However, it cannot be said that the resin composition is satisfactory because it is clouded itself and a dispersion failure such as exfoliation occurs on the surface of a molded article. There is further proposed a resin composition which comprises an aromatic polycarbonate resin, an organic alkali metal salt and poly(phenylmethyl hydrogen siloxane) (refer to Patent Document 6).
There is still further proposed a resin composition which comprises a polycarbonate having a branched structure and an organic metal salt (refer to Patent Document 7). There is still further proposed a resin composition which comprises a polycarbonate having a branched structure, an organic metal salt and a specific siloxane compound (refer to Patent Documents 8 and 9). Although these resin compositions have excellent flame retardancy and transparency, the further improvement of flame retardancy is desired as the use of the polycarbonate is being diversified and products are becoming thinner.
Various flame retardant polycarbonate resin compositions have been developed according to application purpose and vary in flame retardancy level. It is desired that the flame retardancy levels of these materials should be improved as much as possible. For example, if the minimum thickness of a test specimen able to achieve V-0 flame retardancy in the UL94 standard that is widely used as a flame retardancy index for materials for electric applications can be reduced by 0.1 mm, its use as a flame retarding material becomes wide and its effect is extremely large. Even when the flame retardancy level is the same, if the amount of a flame retardant in use can be made as small as possible, this leads to the reduction of a gas generated at the time of processing, the improvement of workability, the stability of quality and the improvement of physical properties all of which are strongly desired.    (Patent Document 1) JP-A 6-306265    (Patent Document 2) JP-A 6-336547    (Patent Document 3) JP-A 9-169914    (Patent Document 4) JP-A 10-139964    (Patent Document 5) JP-B 60-38419    (Patent Document 6) JP-A 2003-147190    (Patent Document 7) Japanese Patent No. 3129374    (Patent Document 8) Japanese Patent No. 3163596    (Patent Document 9) JP-A 2007-31583